Drought and heatwave synergy alters organ-level C:N stoichiometry and carbohydrate dynamics in subtropical tree seedlings

Introduction Frequency and intensity of “hot droughts” are escalating globally, yet the physiological mechanisms governing tree resilience to the synergistic impacts of water deficiency and heatwaves remain poorly understood. Methods Here, we investigated the dynamics of carbon (C), nitrogen (N), and non-structural carbohydrates (NSC) in leaves, stems, and roots of three subtropical tree seedlings ( Schima superba , Cyclobalanopsis glauca , and Phoebe bournei ) during combined stress and subsequent rehydration. Results Our results demonstrate that combined stress triggers a “dual-deficit” mechanism—simultaneous cessation of carbon assimilation and accelerated respiratory consumption, as inferred from the significant depletion of organ-specific NSC pools—that exerts a more profound impact on NSC pools than on C-N stoichiometry. Structural equation modeling revealed a spatiotemporal division of labor where seedlings prioritize acute hydraulic safety by sacrificing foliar metabolic performance, incurring a delayed “metabolic debt” in roots that manifests as significant legacy effects during recovery. Discussion Crucially, the three species exhibited divergent strategies to manage this metabolic capital: S. superba displayed high resilience through “physiological re-priming,” rapidly restoring foliar source capacity; C. glauca adopted a “conservative buffering” strategy, sequestering NSC in stems to bridge the gap between resistance and hydraulic recovery; conversely, P. bournei suffered from a maladaptive “root-leaf decoupling,” where structural rigidity in roots impeded the remobilization of reserves, leading to systemic NSC depletion indicative of potential carbon starvation. These findings underscore that resilience to compound climate extremes depends not merely on static resistance traits, but on the flexibility of inter-organ coordination to resolve the metabolic debts of recovery.